1. Field of the Invention
The present invention relates to a method of controlling low-noise driving of a vibration driven motor used in OA equipment, an AF mechanism for a camera, or the like, and a control apparatus therefor.
2. Related Background Art
Conventional control means used in control of positions, speeds, and accelerations of vibration driven motors are arranged such that their parameters (e.g., phase compensation factors, proportion factors, integral factors, and differential factors) are determined from response characteristics (e.g., step response and frequency response) of the vibration driven motors on the basis of a control theory as in conventional DC motor control means.
When a vibration driven motor is to be controlled by modulating (e.g., amplitude modulation, frequency modulation, phase modulation, or pulse width modulation) an AC voltage applied to a vibration electro-mechanical energy conversion element, modulation frequency components and integral multiples of the modulation frequency component appear in a sideband (i.e., when a frequency bandwidth of the modulation frequency is defined as f.sub.1, the sideband falls within the range from f.sub.0 -f.sub.1 to f.sub.0 --+f.sub.1 or from f.sub.0 -mf.sub.1 to f.sub.0 +mf.sub.1 (m=2, 3, . . .)) of the frequency (f.sub.0) of the AC voltage applied to the vibration driven motor.
In a conventional system having a primary low pass filter in a control loop, high-frequency components cannot be sufficiently attenuated, and an influence of the frequency components falling within the sideband described above typically appears, resulting in inconvenience.
This problem will be described with reference to FIG. 3. Referring to FIG. 3, a vibrator 1 is arranged by bonding a vibrating plate 1-b made of a metal material and an electro-mechanical energy conversion element 1-a made of a PTZ. When an AC electrical signal having a frequency equal to a natural frequency of nth-order vibrations in a direction perpendicular to the surface of the ring of the annular vibrator 1, a resonant displacement occurs in the ring by forcible oscillation, thereby frictionally driving a rotor 2 urged by the vibrator 1.
An electrode pattern (6-wave driving) printed on the electro-mechanical energy conversion element 1-a is shown in FIG. 4. In a conventional method of modulating an AC voltage, an AC electrical signal having a frequency equal to the natural frequency of the 5th-order vibration in a direction perpendicular to the surface of the ring and including the frequency components in the sideband described above is applied to the pattern, the 5th-order (6-wave driving) vibration in the direction perpendicular to the surface of the ring occurs at positions slightly offset from the desired 6th-order (6-wave driving) driving vibration.
Similarly, the (n+1)th- and 7th-order vibrations in a direction perpendicular to the surface of the ring also occur since the electrical signals include the frequency components of the sideband described above. A problem posed by the (n-1)th- and (n+1)th-order vibrations is noise. If the (n-1)th- and (n+1)th-order mechanical vibrations have a displacement of about 0.01 .mu.m and the frequency of the displacement falls within an audible range, the vibration sound can be an audible sound. Even if the frequency does not fall within the audible range (20 kHz or less) but if a difference between or a sum of the frequency of the (n-1)th- or (n+1)th-order vibration and the nth-order vibration falls within the audible range, noise is generated upon contact between the rotor 2 and the vibrator 1.